Chapter 20. RAM technologies

Let’s look
a little more closely at the technology which is used in the various types of
RAM.

In the old days

Back in the
1980’s, DRAM was used. This was dynamic RAM, which was relatively slow. It was
replaced by FPM (Fast Page Mode) RAM which was also dynamic, only a bit
faster.

Originally,
loose RAM chips were installed directly in large banks on the motherboard.
Later people started combining the chips in modules. These came in widths of 8
bits (with 30 pins) and 32 bits (with 72 pins). The 32-bit modules were suited
to the system bus for the 80486 processor, which was also 32 bits wide.

Fig. 135. Older RAM modules.

FPM RAM could not run any faster than 66 MHz, but that was fine for the
system bus clock frequency in the original Pentium processors.

After FPM
came EDO RAM (Extended Data Out). EDO
is a bit faster than FPM because the data paths to and from the RAM cells have
been optimised. The gain was a 3-5 % improvement in bandwidth. The clock frequency
could be increased to 75 MHz, but basically, EDO
is not very different to FPM RAM.

When Intel
launched the Pentium processor, there was a change to using the 64 bit wide RAM
modules (with 168 pins, as in Fig.
127 on page 51, which are still used for SDRAM today.

Fig. 136. An old motherboard with sockets
for both 64-bit and 32-bit RAM modules. From the transition period between EDO and SDRAM.

SDRAM

The big
qualitative shift came in around 1997, when SDRAM (Synchronous DRAM)
began to break in. This is a completely new technology, which of course
required new chipsets. SDRAM, in contrast to the earlier types of RAM, operates
synchronously with the system bus.

Data can
(in burst mode) be fetched on every clock pulse. Thus the module
can operate fully synchronised with (at the same beat as) the bus – without
so-called wait states (inactive clock pulses). Because they are linked
synchronously to the system bus, SDRAM modules can run at much higher clock frequencies.

The 100 MHz
SDRAM (PC100) quickly became popular, and with new processors and chipsets, the
speed was brought up to 133 MHz (PC133).

Another
innovation in SDRAM is the small EEPROM chip called the Serial Presence
Detect chip, which is mounted on the modules. It is a very small chip containing
data on the modules speed, etc.

DDR RAM

It is
expensive to produce fast RAM chips. So someone hit on a smart trick in
1999-2000, which in one blow made normal RAM twice as fast. That was the beginning
of DDR RAM (Double Data Rate). See the module in Fig. 131.

In DDR RAM,
the clock signal is used twice. Data is transferred both when the
signal rises, and when it falls. This makes it possible to perform twice as
many operations per clock pulse, compared to earlier RAM types:

Fig. 138. DDR RAM sends off two data packets
for each clock pulse.

Timings

DDR RAM
exist in many versions, with different the clock frequencies and timings. The timing indicates how many
clock cycles there are wasted, when the motherboard waits for the memory to
deliver the requested data.

With
smaller numbers, we have better timings and the CPU having fewer idle clock
cycles. You may find memory modules of the same clock frequency but with
different timings. The better timing, the more expensive the RAM module is.

Ordinary pc
users need not to speculate in special RAM with fast timing; this is primary
sold to gamers and over-clockers, who tries to achieve the maximum performance
from their motherboards.

Module

Clock frequency

Timing

PC2100

2
x 133 MHz

2-2-2

PC
2700

2 x 166 MHz

2-2-2
2-3-3

PC
3200

2 x 200 MHz

2-3-2
2-3-3

PC
3700

2 x 233 MHz

3-4-4

PC
4000

2 x 250 MHz

3-4-4

PC
4400

2 x 275 MHz

3-4-4

Note that
different timing means that the gain in terms of increased bandwidth doesn’t
quite match the clock speed. It is a bit less.

Next generation RAM

In the
beginning the problem with DDR RAM, was that the RAM modules were poorly standardized.
A module might work with a particular motherboard, but not with another. But
this – which was typical for a new technological standard – is not a big problem
anymore. Intel was initially against DDR RAM. They claimed that Rambus was a
much better design, and that they wouldn’t use DDR RAM. But consumers wanted
DDR RAM, which Intel’s competitors were able to deliver, and in the end even Intel
had to give in. At the end of 2001, the i845 chipset was released, which uses
DDR RAM for the Pentium 4, and later we had the i865 and i875 chip sets, which
use dual channel DDR
RAM.

The next
generation of RAM is the DDR2, which is a new and better standardized version
of DDR using less power. The DDR2 modules operates at higher clock speeds due
to better design with higher signal integrity and a more advanced internal data
bus. The first chip sets to use DDR2 was Intel’s i915 and i925. Later DDR4 is
expected with clock frequencies of up to 1,6 GHz!

Rambus RAM

Rambus
Inc., as already mentioned, has developed a completely new type of RAM
technology. Rambus uses a completely different type of chip, which are mounted
in intelligent modules that can operate at very high clock frequencies.
Here is a brief summary of the system:

The
memory controller delivers data to a narrow high-speed bus which connects all
the RDRAM modules in a long series.

The
modules contain logic (Rambus ASIC), which stores the data in the format
the chips use.

Data
is written to one chip at a time, in contrast to SDRAM where it is spread
across several chips.

The
modules work at 2.5 volts, which is reduced to 0.5 volts whenever possible. In
this way, both the build up of heat, and electromagnetic radiation can be kept
down. They are encapsulated in a heat conducting, aluminium casing.

Rambus RAM thus has a completely new and different design. The modules
are only 16 bits wide. Less data is transferred per clock pulse, but the clock
frequencies are much higher. The actual Rambus modules (also called RIMM
modules) look a bit like the normal SDRAM modules as we know them. They have
184 pins, but as mentioned, the ships are protected by a heat-conducting
casing:

Fig. 139. Rambus module.

As the advanced Rambus modules are quite costly to produce, the
technology is on its way out of the market.